The present invention relates to photocontrols, which are devices for turning roadway, signage, and area lighting on and off, and more particularly, methods for synchronizing timers in photocontrols to the correct local time.
Photocontrols typically use a light level sensor to determine when it is dark enough to turn a light on and light enough to turn the light back off.
There are, however, many situations where it is desirable to turn a light off at a specific time although it may still be nighttime, as well as turn it back on at a specific time. Examples of these needs arise in connection with businesses with extended hours, parking lots, signage and security lighting.
In addition, there has been an increasing interest in saving energy and reducing light pollution. This interest has resulted in the passage of local ordinances that require lighting curfews, which are requirements that certain types of lighting be turned off at specific times.
There presently exist photocontrols that are capable of turning off lighting a fixed number of hours after dusk. For example, some photocontrols (see U.S. Pat. No. 5,895,986) can measure the duration of a night (from dusk to dawn) on a first night and then, the following night, divide that measured duration in half and thus turn the lighting off midway through the night. But neither of these methods can turn off the lighting at a specific time.
Turning off a light at a specific time requires that the photocontrol know the local time. In order to do this, the photocontrol either needs to have an internal clock or to be able to communicate with a remote clock. A photocontrol with a built-in clock has the disadvantage of requiring that the clock be manually set. If this clock setting becomes incorrect for whatever reason, it must be manually reset. Manual resetting usually requires a worker to ascend the lighting pole to reach the photocontrol, which is an expensive and time-consuming task. Furthermore, the necessity of resetting an internal clock of a photocontrol often follows power outages, a change to or from daylight savings time, or as a result of general drift within the internal clock, any of which can require many photocontrols to be reset at the same time. The internal clock may contain a battery to provide power for keeping track of time in the event of a power outage, however, batteries are vulnerable and need to be replaced periodically.
A photocontrol could communicate with a network clock. However, this solution can be very complicated and expensive as the network either needs to exist or be created.
Another existing photocontrol design measures the length of a night and then refers to a programmed table to determine which night of the year has that length. Once it identifies the particular night by its length, given the longitude and latitude of its location, it can estimate the local time. This provides a rough clock, akin to a sundial in accuracy. A principle disadvantage in this method is that a cloudy day, a limb in front of the photocontrol and other circumstances can fool the photocontrol into thinking it is the wrong day, thus rendering the clock even less accurate. Also, should the standard dates for daylight saving time change for whatever reason, such as the passage of legislation by the applicable government, this design will not be able to adapt without reprogramming.
Thus there remains a need for a photocontrol that knows local time accurately but without the disadvantages of prior art solutions.